A white-light-emitting LnMOF with color properties improved via Eu 3+ doping: An alternative approach to a rational design for solid-state lighting

Article abstract of DOI:10.1039/c3cc48344d

The intrinsic white-light-emitting properties of a lanthanide metal-organic framework that approach requirements for solid-state lighting are easily improved by incorporating minute quantities of red-emitting Eu³⁺ into the host framework by virtue of the isostructural character of the La ³⁺ and Eu³⁺ compounds and efficient sensitization of ligands toward Eu³⁺ ions.

Full text of DOI:10.1039/c3cc48344d

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A white-light-emitting LnMOF with color
properties improved via Eu3+ doping: an
alternative approach to a rational design
for solid-state lighting†
Cite this: Chem. Commun., 2014,
50, 1820
Received 1st November 2013,
Accepted 6th December 2013
Yongqin Wei, Qiaohong Li, Rongjian Sa and Kechen Wu*
DOI: 10.1039/c3cc48344d
www.rsc.org/chemcomm
The intrinsic white-light-emitting properties of a lanthanide metal– very rich luminescent emitting sources of metal–organic frame-
organic framework that approach requirements for solid-state works (MOFs),9 there has been great interest devoted to white-light-
lighting are easily improved by incorporating minute quantities of emitting MOFs in recent years. But it is still difficult to target white-
red-emitting Eu3+ into the host framework by virtue of the iso- light-emitting MOFs because the blue and yellow light emitters or
structural character of the La3+ and Eu3+ compounds and efficient blue, green and red light emitters should compensate exactly
sensitization of ligands toward Eu3+ ions.
through the dichromatic and trichromatic approaches, respectively.
Single-phase white light emitters based on MOFs have seldom been
White-light-emitting diodes (WLEDs) have broad applications in achieved.10 On the other hand, a few known examples show good
solid-state lighting (SSL) for the purpose of conserving energy and color rendering properties and high luminous efficiency to meet
miniaturization of equipment.1 At the present time, commercially the criteria for SSL use.11 High-quality white light illumination
available WLEDs are based on the blue InGdN/GaN LEDs encapsu- requires a source with the CIE (the Commission International
lated with a yellow-emitting phosphor (YAG:Ce3+) coating.2 But the ed’Eclairage) coordinate approaching the ideal value (0.333,
WLEDs have drawbacks such as poor color rendering index (CRI) 0.333), CRI above 80, and CCT between 2500 and 6500 K.1,12 To
and high correlated color temperature (CCT) which limit their design a white-light-emitting MOF material according to high-
widespread commercialization in general lighting market. With quality illumination requirements is a great challenge for chemists.
the development of LED chip technology, the shift of emission In this study, we present a novel lanthanide metal–organic frame-
bands of LED chips from blue light to near-UV light can offer higher work (LnMOF) material featuring intense white emission, with
energy to pump the phosphor. There has been great interest shown color properties improved by incorporation of minute quantities
in the employment of near-UV InGaN LEDs with triple-wavelength of red-emitting Eu3+ into the host framework solid.
RGB phosphors to produce WLEDs.3 Unfortunately, the combi-
The material was obtained by the hydrothermal reaction of
nation of different phosphors brings difficulties in the integration La(NO3)3, H2MBDC (isophthalic acid), NaSTP (sodium 4-(2,20:60,200-
of individual materials and may not give rise to uniformly disperse terpyridin-40-yl)benzenesulfonate) with a molar ratio of 1 : 1 : 1 at
and thermodynamically stable compositions. Thus, generating 180 1C for 4 days. Good agreement between the calculated and
white light from a single phosphor to avoid the mixing problems experimental powder X-ray diffraction (XRD) patterns verifies the
is one of the current academic interests.4
purity of the as-synthesized sample. Thermogravimetric analysis
Tremendous efforts have been dedicated to the search for white- (TGA) measurements showed that the title material has excellent
light-emitting materials including metal-doped or hybrid inorganic thermal stability as no strictly clean weight loss step occurred below
materials,5 nanomaterials,6 organic molecules,7 and metal com- 450 1C. The structure was determined using single crystal X-ray
pounds.8 Due to the exceptional tenability, structural diversity and crystallography, studied as [La(MBDC)(STP)] featuring a 3D poly-
meric structure.‡ The La3+ center is in a monocapped square
antiprismatic environment and nine-coordinated to three terpyridyl
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the
nitrogen atoms, one oxygen atom from the sulfonic group of ligand
STP and five oxygen atoms from carboxylate groups of ligands
MBDC. The two metal centers are connected by four carboxylate
groups exhibiting two types of coordination modes (bidentate
bridging and chelating bridging), forming a dimer building block.
As shown in Fig. 1C and D, each dimer building block as a
6-connected node is linked by ligands MBDC to four blocks with
Structure of Matter, Chinese Academy of Sciences, Fujian 350002, China.
E-mail: wkc@fjirsm.ac.cn
† Electronic supplementary information (ESI) available: Details about the pre-
paration of NaSTP and its 1H NMR (DMSO), study on the charge transfer
properties and corresponding quantum chemical calculations, calculation of
photophysical parameters for the Eu(III) compound, XRD, PL. CCDC 969324,
969325 and 974705. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c3cc48344d
1820 | Chem. Commun., 2014, 50, 1820--1823
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550 nm. In addition, the temperature-dependent emission spectra
(Fig. S4, ESI†) showed that the emission intensity becomes weaker
with increase in temperature, but returns to the initial state when the
sample is cooled to room temperature. The stability is attributed to
the dense 3D polymeric structure of the title material.
It is noticeable that the free ligand NaSTP only shows one emission
in solution, but double emission in solid state (Fig. S5, ESI†). By
analysis of the crystal structure of the title compound and the results
of electronic structure calculations (ESI†), we could deduce that the
high-energy emission was due to intraligand charge transfer (ILCT) of
STP and low-energy emission should be relative to p–p attractions
between the parallel ligands STP. The absolute luminescence quan-
tum yields (QY) of the title compound and the free ligand in solid
state measured upon excitation at 370 nm are 11% and 0.8%,
respectively. The enhancement of emission could be attributed to
the planar character via coordination of STP to metal ions which
increases the rigidity of the ligand and thus reduces the loss of energy
by radiationless decay of the intraligand emission excited state.9
Assessment of the white light property for the as-synthesized
material revealed a low CRI, which does not approach the set target
CRI of B90% for SSL use.1 The low CRI is due to the red region
missing in the emission spectra. In order to improve the intrinsic
color properties, we introduced a narrow-band, red emission compo-
nent into the LnMOF system via Eu3+ doping. The notable feature is
that the La3+ and Eu3+ compounds are isostructural, which makes it
feasible to codope them as a uniform compound, resolving some of
the critical integration problems.
Fig. 1 Crystal structure of [La(MBDC)(STP)]: (A) dimer building block, (B)
p–p attractions between the parallel ligands STP, (C) two types of linking
modes between the building blocks, (D) topological representation.
a separation of 10.007 Å, forming a 2D planar structure, and further
linked by parallel ligands STP to another two blocks with a
separation of 16.490 Å, resulting in the dense three-periodic
structure featuring a PCU topology. It is notable that there are
strong p–p attractions between the parallel ligands STP (Fig. 1B).
The outer phenyl rings stack with the central pyridyl rings in face to
face mode (3.639 Å centroid–centroid). The phenyl ring is twisted
with respect to the adjacent pyridyl ring about the interannular C–C
bond by 101, smaller than that observed in related systems.13
PL emission spectra were collected on the as-synthesized solid. It
is remarkable that the title material appears as intense white light to
the naked eye when irradiated with the whole near-UV (350–400 nm)
light, while some of the coordination compounds show only white
emission when excited at certain wavelengths.8a,10b As shown in the
measured emissions (Fig. 2), the title compound shows broad linker-
based emission with two peaks located at around 430 nm and
The pure Eu3+ compound [Eu(MBDC)(STP)] exhibits bright red
photoluminescence emission due to the internal 4f–4f transitions in
Eu3+ (Fig. 3). The narrow-band peaks of Eu3+ at around 590, 614, 650
and 696 nm are attributed to transitions from 5D0 to 7FJ ( J = 1, 2, 3, 4)
respectively. The absence of emissions of ligands MBDC and STP was
evident from the emission spectrum, which is typically diagnostic of
sensitization of ligands toward Eu3+ ions. Calculations showed
that the efficiency of sensitization Zsens is up to 73% (Table 1).14
Fig. 3 Emission spectrum of [Eu(MBDC)(STP)]; inset: excitation spectrum
measured at 614 nm emission.
Table 1 Photophysical parameters of [Eu(MBDC)(STP)] in the solid state
under ligand excitation (373 nm)
Fig. 2 Emission spectra of [La(MBDC)(STP)] excited between 350 and
390 nm; inset: optical image of the white light emission in the powder
sample upon excitation at 370 nm.
tobs (ms)
QY (%)
41
trad (ms)
FLn (%)
Zsens (%)
1.65
2.92
56
73
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SSL application. In general, the Eu-doped samples exhibit
better chromaticity coordinates, higher CRI and cooler CCT
compared with that of the pure La3+ compound, accordingly
with high-quality white light illumination requirements. More-
over, the Eu-doped samples show good emission efficiency.
For example, the absolute luminescence quantum yield of
the 0.10% Eu-doped sample measured under an excitation of
390 nm is 12%, higher than the most of reported white-light-
emitting coordination compounds.10,11a
In the end, it needs to be mentioned that Nenoff et al. have
recently reported broad-band direct white light originating
from an indium compound SMOF-1 and the color properties
approaching requirements for SSL were optimized via Eu3+
doping, opening a new path for the rational design of alter-
native materials for SSL applications.11a Herein, we report a
white-light-emitting LnMOF material with higher luminous
efficiency. The intrinsic white-light-emitting properties can be
easily improved by incorporating minute quantities of red-
emitting Eu3+ into the host framework by virtue of the iso-
structural character of the La3+ and Eu3+ compounds and
efficient sensitization of ligands toward Eu3+ ions.
Fig. 4 Emission spectra of target concentrations of the Eu-doped LnMOF
upon excitation at 390 nm.
The absolute luminescence quantum yield (QY) measured under an
excitation of 373 nm is 41%, which is higher than that of the normal
Eu3+ compounds.9 It is well known that water molecules are often
involved in coordination to meet the high coordination number of
lanthanide ions, which leads to fluorescence quenching.15 The bright
photoluminescence of the Eu3+ compound could be attributed to
beneficial effect of the ancillary ligand MBDC: not only are high-
energy vibrations removed from the inner coordination sphere,
increasing observed lifetime, but the symmetry around the Eu3+ ion
is decreased, rendering the f–f transitions less forbidden, and a better
positioning of the excited state with one of the first ligand STP
increases the efficiency of energy transfer.16
PL spectra were collected for all concentrations of the
Eu-doped LnMOF (Fig. 4). In a typical doping experiment, the
amount of La was kept the same as that for the pristine material.
Because of efficient sensitization of ligands toward Eu3+ ions,
emission of the Eu-doped sample shows red photoluminescence
of Eu3+ ions and has almost no characteristic double emission
peaks of the host framework when the molar ratio Eu/La is
more than 0.2%. Table 2 and Table S1 (ESI†) exhibit color
properties of the 0.02–0.16% Eu-doped samples upon the
excitations at 390 nm and 370 nm, respectively. Apart from
the double emissions of the host framework, narrow-band
emissions of Eu3+ are seen, which become stronger with the
increase of Eu3+ concentration. All of the 0.02–0.16% Eu-doped
samples appear as white light to the naked eye and the color
properties are easily improved to approach requirements for
We gratefully acknowledge the financial support from the
Natural Science Foundation of China (no. 21171165 and
21201165, 81001403).
Notes and references
‡ Crystal data for [Eu(MBDC)(STP)]: C29H18N3O7SEu, Mr = 704.48,
monoclinic, space group P21/c, a = 16.365(4) Å, b = 11.123(3) Å, c =
16.421(4) Å, b = 119.828(3)1, V = 2592.9(10) Å3, Z = 4, Dc = 1.805 g cmꢀ1
,
m(Mo-Ka) = 2.555 mmꢀ1, T = 293(2) K, 20 079 observed reflections, 5886
(Rint = 0.0327) unique reflections and 370 parameters yielded wR2
=
0.1341 and R1 = 0.0408 based on 4896 reflections with I > 2s(I), GOF =
1.050, CCDC 969324; crystal data for [La(MBDC)(STP)]: C29H18N3O7SLa,
Mr = 691.44, monoclinic, space group P21/c, a = 16.490(5) Å, b =
11.144(3) Å, c = 16.625(4) Å, b = 120.16(2)1, V = 2641.5(12) Å3, Z = 4,
Dc = 1.739 g cmꢀ1, m(Mo-Ka) = 1.751 mmꢀ1, T = 293(2) K, 20 273
observed reflections, 5950 (Rint = 0.1036) unique reflections and 370
parameters yielded wR2 = 0.1518 and R1 = 0.0460 based on 3957
reflections with I > 2s(I), GOF = 1.053, CCDC 969325.
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Table 2 Color properties of targeted concentrations of the Eu-doped
LnMOFs upon excitation at 390 nm
Eu/La (%)
CRI
CCT (K)
x
y
0
71
77
84
80
89
93
91
88
94
10 068
7423
6209
5580
5144
5404
5016
4266
4411
0.265
0.293
0.316
0.330
0.342
0.334
0.345
0.373
0.364
0.317
0.346
0.353
0.378
0.368
0.355
0.364
0.387
0.364
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
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